Fission (分裂 Bunretsu) is the ability for a single being to split into two different copies of himself, divided more or less equally in terms of power and appearance, but usually share a single lifeline. It is also the reversal and opposite of a fusion.
When Goku first meets Kami, he explains he and King Piccolo were once a single being that ventured to the Guardian of Earth and eventually attempted to succeed the then-current Guardian whose end was approaching. The Guardian, however, saw evil within this Nameless Namekian and turned him away. Refusing to give up, the Nameless Namekian dedicated himself to purging the evil within himself, thus creating Kami and King Piccolo, the Nameless Namekian's respective good and evil counterparts.
The Nameless Namekian is the only Namekian in the entire series shown to perform this, but there are two different and conflicting versions of the event. The Nameless Namekian was portrayed as consciously expelling a black ghost-like apparition (presumed to be King Piccolo). But later, when Grand Elder Guru probes Krillin's mind he sees the Nameless Namekian suddenly falling out of bed and onto the floor, at which point King Piccolo tears away from his body, making the divided beings. One explanation to these conflicting versions is that the original flashback was actually an event from within the Nameless Namekian's subconscious, since he appeared to have been sleeping during the start of Guru's vision, which is usually the more well known of the two scenes.
A third version, similar to the first one, later appears in Kami's flashback.
One interesting attribute of Namekian Fission is that the two Namekians produced by fission are life linked, thus if one dies the other will die too, unless one reincarnates themselves as King Piccolo did when he gave birth to Piccolo which caused the life link to be transferred to Piccolo, thus Kami survived. The life linked death occurs whether the death was natural (such as old age) or unnatural (death in battle or other unnatural causes). It should also be noted that the life link makes one half killing the other half essentially a suicidal action which is why Kami was unable to stop King Piccolo himself and why he attempted to seal Piccolo Jr. away with the Evil Containment Wave as it would allow him to defeat Piccolo without killing him, though after it failed he later decided to kill Piccolo Jr. even if it meant his own death, though Goku managed to persuade him not to go through with it. Additionally if one is revived the other will be revived as well as Kami came back to life after Piccolo was resurrected. However, the life link is negated if the two fuse which essentially undoes the fission.
After Van Zant and Smitty shot Bee and later Mr. Satan, Innocent Buu becomes more enraged than he has ever been and he uses fission to expel the evil from his body in order to keep control. The evil leaves Buu in the form of a giant cloud taking most of his power with it, giving birth to a tall, thin, and gray version of himself called Evil Buu. The "evil" Majin Buu is a polar opposite of the "good" Majin Buu in all aspects apart from apparel (except for him having a dark blue cape where Good Buu has a purple cape), including the color of his flesh (a drab gray), his incredibly thin body, and gravelly manner of speech.
A battle soon ensues between the two Majins, with the evil incarnation of Buu quickly gaining the upper hand and consuming Good Buu after reflecting his Transfiguration Beam. Evil Buu then eats the chocolate Good Buu, causing them to re-fuse, this time with the evil on the outside instead of the inside, resulting in Super Buu.
Effects and discrepancies
It appears that if one of these two Namekians of the division gives birth, the offspring can end up mutated and malformed. This is shown by King Piccolo, who spawned a large number of monstrous Namekians. However it is possible that this was a result of King Piccolo's evil nature and not the fission itself.
When a Namekian splits himself into two separate beings, one good and one evil, they share a single life and are life linked. If one should perish of natural or unnatural causes of death then the other will die within brief seconds. Before King Piccolo dies, he inadvertently saves Kami's life by spawning a final son to avenge his death. This new offspring, Piccolo Jr., contained his father's essence (including his techniques and memories) and shared his lifeline with Kami as his father once did, and is essentially King Piccolo's reincarnation. It was not until much later, during the fight with the Saiyans when Piccolo Jr. was killed, that Kami was actually shown dying as a result of this life link. However, if one is resurrect the other will be revived as well.
There are some minor discrepancies to the physical appearance of the two new beings in question. Just after Guru's vision, there is a brief shot of Kami and King Piccolo standing side-by-side in their youthful states and they are virtually identical in size and appearance. Early on, however, it is shown that King Piccolo dwarfed everyone he came into contact with, yet Kami seemed to be of a much more reasonable size, though it is also never confirmed at any point if King Piccolo is taller than Kami if both are at the same stage in life (at one point King Piccolo wishes for restored youth and increases in size thereafter). King Piccolo, in his ancient state, was portrayed with a much more bulbous head than Kami, though this could actually be attributed to laying eggs (King Piccolo had already given birth twice in his aged state when introduced).
It is shown that when a Namekian splits into two their power levels plummet, since King Piccolo and Kami had very low power levels separately, but together they could be as powerful as a Super Saiyan, as said by Grand Elder Guru. Also, when Piccolo Jr. joined with Kami, he was much more powerful than an unmastered Super Saiyan and his power was roughly equal to that of Android 17.
Video Game Appearances
In Dragon Ball Xenoverse, during Elder Kai's training of the Future Warrior, Elder Kai uses an unknown form of fission on the Future Warrior to create a doppelganger of the Future Warrior. This doppelganger possesses the Warrior's skills, appearance, and voice. However unlike the Future Warrior, their doppelganger is capable of using Villainous Mode. Upon defeating it, Elder Kai reveals that the doppelganger is a representation of the Warrior's ambitious Ego that was growing within the Warrior's heart, thus the Ego doppelganger was an evil manifestation born of the Warrior's growing Ego, similar to King Piccolo & Evil Buu. Elder Kai's Fission technique differs from other forms of Fission in that it is initiated by someone to cause fission to occur in another person. This technique apparently works on all the Warrior's possible races (Saiyan, Human, Frieza's race, Namekian, and Majin) even if they are of a race normally incapable of fission such as a Saiyan, Human, or Frieza's race.
In Dragon Ball Xenoverse 2, inside a time rift that had formed around Majin Buu's house, Majin Buu is inspired by Mr. Satan's familial relationship with his daughter Videl and decide to create his own offspring using fission. The Future Warrior can assist him by providing Majin Buu with food which gives him energy necessary to perform fission. His fissions result in the creation of smaller Majins that resemble Fat Buu. If the Future Warrior is a Majin then the number of children he can create with fission is increased to 10.